The present invention is directed to a phase separator for use in an ultra high vacuum system, for example, a molecular beam epitaxy (xe2x80x9cMBExe2x80x9d) system and, more particularly, to a phase separator integrated into a cryogenic reactor chamber within the MBE system that facilitates the smooth flow of liquid nitrogen into and gaseous nitrogen out of the system.
Ultra high vacuum systems are used in many manufacturing, scientific and other applications. Throughout this application, ultra high vacuum (xe2x80x9cUHVxe2x80x9d) systems are defined as those having base system pressures less than approximately 10xe2x88x928 Torr. One example of a system employing UHV is epitaxial crystal growth.
One such epitaxial crystal growth application employing UHV is molecular beam epitaxy (xe2x80x9cMBExe2x80x9d). In MBE, thin films of material are deposited onto a substrate by directing molecular or atomic beams onto a substrate. Deposited atoms and molecules migrate to energetically preferred lattice positions on a heated substrate, yielding film growth of high crystalline quality and purity, and optimum thickness uniformity. MBE is widely used in compound semiconductor research and in the semiconductor device fabrication industry, for thin-film deposition of elemental semiconductors, metals and insulating layers.
Purity of the growing films depends critically on the operating pressure of the growth chamber and the residual gas composition. To ensure the high level of purity required, for example, by the semiconductor industry, the MBE growth chamber base pressure is necessarily in the ultra high vacuum range (UHV), typically less than 10xe2x88x9210 Torr.
Furthermore, film growth rates, film composition and film doping levels depend critically on the operating temperature of numerous critical components of the growth system, for example, the source cells and the substrate carrier. To this end, MBE growth chambers often employ a liquid nitrogen filled cryogenically cooled shroud (xe2x80x9ccryo-shroudxe2x80x9d) surrounding interior elements and enclosing the active growth region. This cryo-shroud serves a multiplicity of purposes: 1) to enhance the level of vacuum in the UHV chamber by condensing volatile residual species not removed or trapped by the vacuum pumping system i.e. providing a cryo-pumping action, 2) to enhance the thermal stability and temperature control of critical growth reactor components, and 3) to condense and trap source material emitted from the effusion cells but not incorporated into the growing film.
The implementation of a liquid nitrogen filled cryo-shroud in an UHV system requires a phase separator that allows the escape of gaseous nitrogen generated by the vaporization of the liquid nitrogen as heat is absorbed by the cryo-shroud. The phase separator also enables a replenishing feed of liquid nitrogen into the cryo-shroud to maintain the desired operating temperature. A conventional implementation of such an external phase separator is shown in FIG. 1.
As shown in FIG. 1, vacuum chamber 100 contains a cryogenic shroud 110 having a liquid nitrogen inlet 112 and a liquid nitrogen outlet 114. A phase separator 120 is connected to inlet and outlet 112, 114 via ports 132, 134 and lines 122, 124, respectively. Liquid nitrogen at or below its atmospheric boiling point of 77.5xc2x0 K. (xe2x88x92195.5xc2x0 C.) is introduced into phase separator 120 via inlet 142 and flows through port 132 and line 122 and enters cryo-shroud 110 via inlet 112. As nitrogen in cryo-shroud 110 warms to the boiling point due to heat absorbed from vacuum chamber 100, vapor forms within the body of the liquid and bubbles rise by gravity to the top of the cryo-shroud and ultimately out through outlet 114, liquid-filled exhaust line 124, port 134 and gaseous nitrogen escapes via exhaust 144. The formation and flow of these vapor bubbles result in the turbulence and seething normally associated with boiling action, causing mixing effects with the liquid-state nitrogen and counteracting the natural tendency for colder, more dense liquid to settle into the lower portion of the cryo-shroud.
Several problems are associated with a conventional phase separator design. First, the small cross-sectional area of the exhaust line results in a flow restriction for the vapor bubbles and formation of a xe2x80x9cfrothingxe2x80x9d, boiling region in the upper section of the cryo-panel. This region will be elevated in temperature above the liquid nitrogen boiling point, resulting in poor heat absorption from the adjacent cryo-shroud surface. Second, large pockets of gas can accumulate within the body of the cryo-shroud before ultimately breaking loose and flowing to the exhaust line, giving rise to local, temporary warming of the cryo-shroud surface at the location of the trapped gas pocket. Third, the configuration results in an operating pressure within the cryo-shroud considerably above atmospheric pressure. This causes an elevation of the liquid nitrogen boiling point and an overall rise in the operating temperature of the cryo-shroud. A temperature rise of even a few degrees can significantly degrade the cryo-pumping performance of the cryo-shroud. For example, the vapor pressure of carbon dioxide (CO2) increases exponentially with temperature from 10xe2x88x929 Torr at 72.1xc2x0 K. to 10xe2x88x927 Torr at 80.6xc2x0 K. The limited surface area of the gas-to-liquid interface in the exhaust line enhances these problems.
The present invention overcomes the above-difficulties by integrating both the phase separator and the cryo-panel within the vacuum chamber, thus eliminating the lines of relatively small diameter connecting the vacuum chamber to an external phase separator. According to the present invention, a cryogenic panel disposed within a vacuum chamber, e.g., an MBE reaction chamber, includes a cryogenic shroud region and a phase separator region. Liquid nitrogen is introduced into the cryogenic panel via an inlet line. As the liquid nitrogen warms and vaporizes, nitrogen vapor rises within the shroud. The phase separator region within the cryogenic panel provides a large area vapor-to-liquid interface held at near atmospheric pressure, ensuring that nitrogen vapor may escape the panel smoothly, without forming gas bursts, and with minimal turbulence and general disturbance of the liquid reservoir.
The upper end of the cryogenic panel containing the phase separator region preferably is vacuum jacketed. The liquid nitrogen feed mechanism is designed such that the liquid-to-vapor phase boundary is always held at a level within the region encompassed by the vacuum jacket. This prevents exposed external surfaces of the cryo-shroud from varying in temperature from the nominal 77.4xc2x0 K. associated with the internal liquid nitrogen bath, thereby optimizing its performance and thermal stability.
The flow of liquid nitrogen into the cryogenic panel will generally be intermittent, gated by a level sensor located within the phase separator and a fill mechanism. Following a significant time period of no flow, the feed lines will have warmed considerably and liquid nitrogen may vaporize initially as it flows from the bulk supply tank to the cryogenic panel. This can give rise to high velocity gas injection into the phase separator region and mixing effects between the vapor and liquid phases, until the delivery line has sufficiently cooled. Normal flow of liquid nitrogen into the system can have similar, although less severe, effects. Terminating the inlet line in a xe2x80x9cshower headxe2x80x9d arrangement that disperses the gas or liquid flow and directs it in a generally horizontal direction can minimize turbulence in the liquid nitrogen reservoir, and general disturbance of the vapor/liquid interface.